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Dengue is the most important vector-borne viral disease in tropical countries, with at least 100 million cases reported each year.1 In Brazil, infection by the dengue virus has increased significantly in the last two decades after the reintroduction of mosquito vector Aedes (Stegomyia) aegypti (L.). Sympatric presence of Ae. aegypti and Aedes (Stegomyia) albopictus (Skuse) generates a new problem for dengue epidemiology. Aedes albopictus was first observed in Brazil in 1986, in the states of Espírito Santo, Minas Gerais, Rio de Janeiro, and São Paulo.2 Currently, this species is found in several states in this country. In some municipalities of Minas Gerais that reported Ae. aegypti in previous years, health workers have observed that Ae. albopictus is now the principal mosquito species.3 Aedes albopictus dissemination is due mainly to the international shipping trade of used tires, which provides an ideal mechanism for passive dispersion of immature stages.4,5
The presence of Ae. albopictus is a serious public health problem. First, it plays an important role in the transmission of several arboviruses and its susceptibility to these viruses is even greater than that of Ae. aegypti.6 Second, the species maintains the possibility of vertically transmitting serotypes 2 and 3 of the dengue virus naturally.7 Additionally, the mosquito shows aggressive anthropophilic behavior and a great adaptability to different habitats.8,9 This success is perhaps due to extreme variation in its adaptative traits, such as diapause10 and hardiness to cold temperatures.11
Epidemiologic surveillance associated with vector control remains the only way to prevent dengue outbreaks since an effective vaccine is not available.12,13 Surveillance is conducted by the determination of different indices that take into account the number of immature and mature forms of the mosquito. In Brazil, the current control program applied by the National Health Foundation/Ministry for Health for Ae. albopictus and Ae. aegypti populations is a set of concomitant and integrated epidemiologic surveillance, vector elimination, and social, environmental, and medical assistance. There are many problems that compromise the reliability of entomologic surveys. To obtain these indices, it is necessary to enter houses. In localities with 
5,000 houses, 10% need to be surveyed (National Health Foundation, 2001).14 These methodologies require time, money, and trained personnel. However, it is occasionally difficult to enter houses, which decreases the significance of the index.
An interesting alternative is the use of a distinct index known as the Premise Condition Index (PCI).15 This tool relates the condition of property, such as the house and the yard, and the degree of shade, to the occurrence of Aedes sp. oviposition. The calculation of the PCI is achieved by the association of the three property variables (house, yard, and shade) calculated in scores from 3 to 9. The lowest score indicates property in good condition and an unfavorable breeding environment, while the highest score indicates property at high risk for infestation with to Aedes sp. The present study tested this instrument in an urban area of Botucatu, Brazil to confirm its effectiveness.
With the assistance of the Municipal Secretary for Health of Botucatu, we divided the city into 105 quadrants that covered its entire area. Houses (one per quadrant) were randomly chosen at the time of the visit. House conditions were recorded according Tun-Lin and others.15 After choosing and qualifying the house, one ovitrap was set up, with the home owner’s consent, in a peridomociliar environment, preferably in the garden in a shady place. When this was not possible, the ovitrap was placed in the patio. An ovitrap consisted of a black plastic container filled with water, with a wooden stick to collect eggs. Eggs from positive traps were induced to eclosion in the laboratory. Differentiation between species was carried out on adult forms by thoracic markings according to morphologic keys of Forattini.16 For the purpose of finding a significant association for the three property variables (house, yard, and shade), PCI scores (3–9) were compared with the presence or absence of Aedes eggs.
The occurrence of Ae. albopictus eggs showed a uniform distribution all over the city (41 of 105 ovitraps). Two ovitraps had only Ae. aegypti eggs, and 11 ovitraps had eggs from both species. When houses were grouped according to similar PCIs (Table 1
), 65% of those with scores of 8 and 9 (considered to be houses in the worst condition) were positive with the presence of eggs, while only 19% of the properties with scores of 3 and 4 (houses in excellent condition) were positive. There was a positive correlation (r = 0.9684, P < 0.01) between house condition and percentage of positive houses, which clearly showed the usefulness of this method.
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Ovitraps have been successfully used for detection of Aedes sp. in many countries.17 In Brazil, superior performance has been shown in comparison to larval surveys.18 Braga and others17 showed that that in 2,944 houses inspected, 7.5% were positive by larval survey while 25.1% were positive by ovitraps. Thus, to determine the degree of infestation of this vector in the city, the trap method was used in our study. Unexpectedly, the predominant mosquito was Ae. albopictus, not Ae. aegypti.
Aedes albopictus was found all over the city, including the downtown (urban) area. This is an important finding for municipal epidemiology because Ae. albopictus has been shown to maintain a dengue epidemic.19 Although it is not considered important in virus transmission to humans, this species is able to sustain the virus in nature due to high rates of trans-ovarian transmission.20 This reinforces other hypotheses21–23 that the virus in semi-rural, rural, and forest areas could be sustained by a vector more efficient than Ae. aegypti by combining transovarian transmission and periodic presence in human and primate populations.
The present study showed the effectiveness of the PCI because worse conditions in houses showed a higher correlation with the presence of Aedes mosquitoes than in well-maintained houses. However, further studies should be undertaken to improve the technique, essentially to obtain a clearer distinction between intermediate and high PCI scores. Gómez and others24 also tested the PCI and showed its accuracy with other factors related to mosquito breeding. This does not mean that if sufficient resources are available, houses with lower PCI scores should be neglected. However, it does provide guidelines for locating houses that have been shown to contain most of the eggs. The major advantage of the PCI is that it offers a rapid assessment method for selecting houses for survey. Conversely, the PCI method requires specific training of health workers, since estimation of index parameters is reasonably subjective. In the case of dengue outbreaks, by having all representative house indices of the region, it will be much easier and less expensive to control the epidemic.
Received April 2, 2004. Accepted for publication February 24, 2005.
Acknowledgments: We thank the Municipal Secretary for Health for their help in collecting field material. Paulo Eduardo Martins Ribolla is a fellow of the Conselho Nacional de Desenvolvimento Científico e Tecnológico.
Financial support: This study was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (Proc. No. 01/06647-9).
* Address correspondence to Paulo E. M. Ribolla, Departmento de Parasitologia, Instituto de Biociências, Universidade Estadual Paulista, Botucatu, São Paulo, Brazil. E-mail: pribolla{at}ibb.unesp.br 
Authors’ addresses: Letícia A. Nogueira, Letícia T. Gushi, Newton G. Madeira, and Paulo E. M. Ribolla, Laboratório de Entomologia Molecular, Departamento de Parasitologia, Instituto de Biociências, Universidade Estadual Paulista, PO Box 510, 18618-000, Botucatu, São Paulo, Brazil. João E. Miranda, Equipe do Controle de Zoonoses, Secretaria Municipal da Saúde, Rua Major Matheus, 07 Vila dos Lavradores, 18609-083, Botucatu, São Paulo, Brazil.
Reprint requests: Paulo E. M. Ribolla, Departamento de Parasitologia, Instituto de Biociências, Universidade Estadual Paulista, Botucatu, São Paulo, Brazil, Telephone: 55-14-3811-6239, Fax: 55-14-3811-6239, E-mail: pribolla{at}ibb.unesp.br.
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